Conventionally, EMD is produced by electrolysis of a manganese-containing electrolyte, such as a 0.5 to 1.2 mol/liter solution of manganous sulphate using titanium, a lead alloy or carbon as anode. The reaction scheme is:
______________________________________ Anode: Mn.sup.2+ + 2H.sub.2 O .fwdarw. MnO.sub.2 + 4H.sup.+ + 2e Cathode: 2H.sup.+ + 2e .fwdarw. H.sub.2 Overall: Mn.sup.2+ + 2H.sub.2 O .fwdarw. H.sub.2 + MnO.sub.2 + 2H.sup.+ ______________________________________
An adherent layer of EMD is deposited on the anode and when the thickness of the deposit reaches 10-30 mm or more, the anode is pulled out of the cell and stripped by impact, i.e. by hammering or even using an explosive. After cleaning and reactivation when necessary, the anode is replaced in the cell and electrolysis is resumed. The EMD obtained is in the form of a large, hard block which is dried, crushed, washed and submitted to further treatment to end up with a powder suitable for use as a dry cell depolarizer.
As pointed out in U.S. Pat. No. 3,855,088, in such a process the anode is taken out of the cell every 10 to 40 days for removal of the EMD deposit, but the cathode is used continuously. Because of impurities in the electrolyte, a cluster builds up on the cathode and lowers the efficiency of the process, so that operation had to be interrupted every 5 to 6 months for removal of the strongly-adherent cathode cluster. Accordingly, U.S. Pat. No. 3,855,088 proposed to facilitate removal of this cluster by, every 6 months or so, connecting the cathode as an anode for about 3 to 10 hours to deposit MnO.sub.2 on to the cluster. This was found to reduce adherence of the cluster, which could then be removed, without damage to the electrode, by pulling it out of the electrolyte and applying an impact in the same way as the block of EMD is removed from the anode.
It has long been recognized that the discontinuous "batch" process involving the stripping of anodically deposited EMD has serious disadvantages, including the difficulty of grinding and otherwise processing the blocks of EMD to a suitable particle size for use as a battery depolarizer. Also, removal of the deposit and reactivation cause damage to the anodes and these must be replaced after several months. Furthermore, the electrodes must be spaced apart by a distance which is sufficient to accommodate the EMD deposit and, as a consequence of the resulting high cell voltage, the process cannot be economically operated at current densities above about 200 A/m.sup.2.
One unsuccessful attempt to obviate these disadvantages was the electrolysis of manganous nitrate using alternating current to produce particulate EMD directly in the electrolyte, without it being deposited on and removed from the anode.
Another proposal, described in U.S. Pat. No. 3,065,155, was to carry out electrolysis of an aqueous acidic solution of manganous sulphate, while agitating the electrolyte and maintaining a given concentration of Mn.sup.3+ ions in a manner to precipitate particulate MnO.sub.2 directly in the electrolyte. However, the electrolyte (containing about 350 g/l of sulphuric acid) is very corrosive, which necessitates thorough and expensive washing, and the efficiency is lower than with the conventional batch process.